Your instincts ( or thoughts) are pretty much correct, only, not exactly for the reasons you might think they are....
As someone who has made that "crossover" from being a single to becoming a parent I can sincerely tell you that , Although I am male, meaning that I did not go through pregnancy and hormonal changes, the "me" from before my son was born ( we have a baby daugheter coming in a few weeks) has almost nothing to do with the "me" that exists now, and I am honestly glad that the previous "me" is not around any more...
I didn't marry young... I met my wife when I was 35, and we got our son when I was 41 ( almost 3 years ago) - in the meantime I was having lots of fun and watching as my m frineds, one by one, joined the "parents" club and , "changed" as it appeared to me at the time.... but then I also became one of them
It is not that we changed... ( or mabye it is, but the change comes from additional wisdom) ... but once you are a parent your priorities, your outlook, even your hopes and dreams take a completely different dimention- a dimetion that individuals who are not part of the "club" can not even begin to imagine- for the simplest reason of all.... We are not thinging only about ourselves any more .
Once we have a child to take care of... one of the first things that happens ( after the initial fear and anxiety) is that one realizes how empty, meaningless, and downright stupid most of the things one cared about as a single truly are. ANother thing you realize is the true importance of other people in your life- and the way they affect it... it is as if you are watching a play in the theatre and, at some point, a curtain comes up and you realize you were only seeing a part of the stage , and it is the part where the most un-important things were happening. We only thought they meant what they meant because we had no idea what was goind on on the rest of the stage and what was it all about.
By virtue of being one of the last people in my crew to become a parent, but still having a few friends who are not, I had the same experience with my friends as you do.... It is not that we want you to join our "club" so that we can all be friends again ( we are friends) ... it is because we want the ones we care about to rise up to another level- the one that can be fully understood only if you do have those responsibilities, and the knowldge that comes with the territory.... for it is only that knowledge which would make most people who are not parents understand why/how a discussion about brestfeeding and the best brand of dipers can be more interesting and fulfilling than a conversation about a recent club hook-up, a "perferct career" plan , what did he/she say about me when drunk, or the gazillion useless things ideas and characters we fill our days with while we try to give meaning to our pre-parental existence.
So, again,in a way you are right when you say that we ( the freinds who became parents) have "crossed over" - and a gap between the existences has opened.. ( BTW, it will only get larger s the time passes.. or until you "join the club") .... and Yes, your friends, at least the ones that truly care about you, do want you to join in... for, no matter how it might look from the outside- becoming, and being a parent gives one a meaning and purpose that nothing else can- it also gives the kind of hapiness that those who are not can never comprehend. So, they want you , as their friend, to share in that hapiness , to be able to see the world and understand it, in a way in which those who have to think only of themsleves can never come close to imagining.
As previously stated.... I did pretty good before I became a parent ( If I may say so myself) but now l find some of the thinks I believed and dealt with before utterly funny and stupid, while I realize that most of them ( although fun and interesting , according to most descriptions) were nothing but weak attempts to fill time or justify my existence ( to myself, if to no one else) .... But the world only opened up to me once I crossed over from being a child to being a parent .. and do not get me wrong, we are either one or the other.. you may be whichever age you like, but until you have taken the responsibility of children upon you... you are a child yourself, and a very selfish one - only most of us fail to see it, just like any child would.
I sincerely hope your wishes come true and you become a parent soon.. I know it seems scary and difficult and uncertain... as if it is some kind of an end to our existence- but, the fears and anxienties dissapear incredibly fast and afterwards, you will be left with the realization that it wasnt an end, but just a beginning of your real existence and that everything before was, at best, a way to kill time while waiting for the curtain to come up so that you can see the whole stage.